[development] Improving theme('username') with inspiration from Facebook

[Chris Johnson]

I don't know that all of the Facebook options are useful to us, and it
seems like a couple might problematic for localization and
internationalization.  Still, it would be nice to have a some
flexibility in styling user names in core.

How about something like pluggable "theme-lets" or "micro-themes," so
site owners can pick and choose which things they want special theming
applied to, without getting a huge barrel full of unwanted extra code,
sort of like enabling additional types in CCK?

(Simply reading and interpreting the PHP code is a major bottleneck
for any site without a PHP accelerator/code cache of some kind,
especially with a large number of modules enabled.  Thus, I favor
finding ways to optionally include code.)